1. Field of the Invention
The present invention relates to electronically commutated electric machines in which the commutation is carried out with the aid of simple and cost-effective rotor position sensors.
2. Description of the Related Art
Electronically commutated electric machines, in particular electric motors, in combination with rotor position sensors have the advantage that, in contrast to sensorless methods for determining the rotor position, such as the so-called back EMF method, they allow the electric motor to be immediately started with a maximum torque, and allow this torque to be maintained even when a rotor is locked.
In three-phase electric machines, the rotor position sensors that are usually used have three sensor elements, for example Hall sensor elements, which detect the field of a radially oriented sensor magnet situated on the rotor of the electric machine, i.e., the stray field of permanent magnets situated on or in the rotor. Optical methods using so-called sensor disks may also be used for this purpose.
For cost reasons, the rotor position sensors have the simplest possible design, and therefore have only a low resolution. As a rule, the minimum resolution must correspond to 360° divided by the number of phases m and by the number of pairs of poles p of the rotor of the electric machine. For the commutation, the stator coils of the electric machine are controlled as a function of the detected rotor position, a commutation generally occurring when the rotor position signal changes. The rotor position sensors are usually oriented in such a way that a control unit, which carries out the control of the stator coils as a direct function of the rotor position signal, controls the stator coils in such a way that the stator magnetomotive force (stator magnetization) on average is oriented perpendicularly with respect to the rotor magnetomotive force (rotor magnetization). The angular range of the active current feed is 360° divided by the product of the number of phases and the number of pairs of poles. For a two-pole machine, this results in an angular range of 60°, and therefore, six commutation operations per rotor revolution.
Since the torque of an electric machine is proportional to the vector product of the electrical magnetomotive force and the exciter flux density, the maximum torque is generated at an electrically effective angle of 90° between the stator magnetomotive force and the rotor flux density. The electrically effective angle is computed from the mechanical angle divided by the number of pairs of poles of the electric machine. Thus, to generate the maximum torque, an electrically effective angle of 90°, averaged over time, must be present between the magnetomotive force and the exciter flux density. A stator coil is thus connected to the voltage source at the exact point in time when its magnetomotive force axis has an electrically effective angle of 90° plus an electrically effective angle of 60° (against the rotational direction), which results from one-half the width of a commutation interval, relative to a magnetization axis (D axis) of a pair of poles of the rotor, and is disconnected from the voltage source at an angle of 90° minus an electrically effective angle of 60° (against the rotational direction) which results from one-half the width of a commutation interval.
This type of control provides good results for electronically commutated electric machines having magnetically symmetrical rotors, and is usually used in particular when the electric machine is to operate in both rotational directions and over a wide rotational speed range.
If the electric machine has a rotor with embedded magnets, instead of the usual design having shell- or ring-shaped surface magnets, the common square or loaf-shaped magnets are present inside the rotor yoke. This results in a magnetic asymmetry of the electric machine, since the permeance in the direction of the magnetization (D axis) is less than in a direction transverse thereto (Q axis). This results in an inductance of the electric machine which is a function of the rotor position. The stator coil, whose magnetic axis coincides with the D axis of the rotor, has the minimum inductance, and the phase conductor, whose magnetization axis coincides with the Q axis of the rotor (which is offset by a 90° electrical rotor position with respect to the D axis), has the maximum inductance.
For these types of electric machines having rotors with embedded permanent magnets, the above-mentioned type of control is not optimal. At the switch-on time of a phase conductor, its inductance is low, subsequently reaches the maximum value when the stator magnetomotive force is oriented perpendicularly with respect to the rotor magnetomotive force, and subsequently drops once again. The drop in the inductance at the switch-off time of the phase conductor causes a sharp rise in current in this phase conductor, resulting in an intense load on the semiconductor circuit elements and which is thus also associated with high switching losses. The resulting current pattern for this type of electric machine has an effective value of the current, which is high in relation to the average value of the current, which is largely responsible for the losses.
A simple remedy results from an early commutation, which is carried out, for example, by rotating the rotor position sensor by a defined angle against a predetermined rotational direction. The increase in current during switching, and thus the load on the semiconductor circuit elements as well as the switching losses, may be reduced in this way. However, this is applicable only for electric machines which are operated in only one rotational direction. On the other hand, if the electric machine is to be operated in both rotational directions, symmetrical precommutation may thus be achieved in both rotational directions. When the rotor position sensor on the electric machine is rotated to achieve a lead in one rotational direction, this always results in a lag in the opposite rotational direction.
According to the present related art, this precommutation is initiated on the one hand by using rotor position sensors having a much higher resolution than for the rotor position sensors according to the minimum requirements for the resolution, i.e., a resolution of 360° divided by the product of the number of phases and the number of pairs of poles. On the other hand, an adjustable delay element is used which achieves the desired precommutation as the result of a delay in the commutation signal, which is a function of the rotational speed. The delay element may also be implemented as a software routine in a microcontroller. However, both measures increase the complexity of circuitry and programming, thus reducing the reliability.
The object of the present invention, therefore, is to provide a system for operating an electric machine, a motor system, and a method for operating an electric machine which allows simple implementation of an operation of an electronically commutated electric machine, having magnetically asymmetrical rotors, in both rotational directions.